1. Field of the Invention
The present invention relates to a pressure transducer which employs a semiconductor piezoresistive effect to convert stress to an electrical signal, and more particularly to a .pi.'.sub.63 transducer where force is applied perpendicularly to the crystal face of a silicon single crystal and a voltage output corresponding to the applied force is taken from the direction perpendicular to the direction in which the current flows.
2. Description of the Related Art
Conventional structures of force transducers employing the semiconductor piezoresistive effect as a detection method consist of structures where force is applied to a semiconductor such as silicon or germanium via a medium, and structures where force is applied to a semiconductor directly. Also, detection methods such as the following have conventionally been employed; deriving an output voltage from the change in resistance in the aforementioned semiconductor due to the force applied; incorporating this change in resistance into a wheatstone bridge circuit and deriving an output voltage from that circuit and; a method known as the .pi.'.sub.63 method whereby the direction of the electrical current flow, the direction of the voltage detection and the direction of the applied force are all at right angles to each other.
The force transducer in the present invention is of the kind which employs the .pi.'.sub.63 method of piezoresistive effect.
FIGS. 9A and 9B show an example of a force transducer for the related art where the current, voltage and force are at right angles to each other (U.S. Pat. No. 4,833,929). The construction of the force transducer 1000 is as follows. A force transmission block 5 which is square shaped in the horizontal cross-sectional plane is mounted on the upper (110) face 1a of the silicon single crystal structure 1, and the lower (110) face 1b of the silicon single crystal structure 1 is in turn mounted on a support bed 4. Then, a pair of input electrodes 2a and 2b and a pair of output electrodes 3a and 3b are formed on the upper (110) face 1a of the aforementioned silicon single crystal structure 1. The input electrodes 2a and 2b are formed at an angle of 135.degree. going away from the &lt;001&gt; axis towards the &lt;1-10&gt; axis, and the output electrodes 3a and 3b are in turn formed at an angle of 90.degree. from the aforementioned input electrodes 2a and 2b.
In this force transducer 1000, force is applied at right angles to the upper (110) face 1a of the aforementioned silicon single crystal structure 1 via the aforementioned force transmission block 5. The output from the force transducer for the corresponding force is then taken as the voltage between the output electrodes 3a and 3b which are at an angle of 90.degree. from the input electrodes 2a and 2b.
With the aforementioned force transducer 1000, the intention is to produce an output which is dependant upon the perpendicular stress .sigma..sub.3 (.sigma..sub.&lt;110&gt;) generated along the axis &lt;110&gt; as a result of the force applied along the &lt;110&gt; axis perpendicular to the (110) face 1a of the silicon single crystal structure 1. Assuming that the support bed 4 is rigid, the acquired output voltage is dependant upon the stress .sigma..sub.3 generated along the &lt;110&gt; axis by the applied force, and the output voltage .DELTA.V' for this case is given by equation 1. EQU .DELTA.V'=b.multidot..rho..multidot.J.multidot..pi.'.sub.63 .multidot..sigma..sub.3 1
In equation 1, b is the length of the force receiving face of the force transmission block along the direction of the output electrodes, .rho. is the resistance ratio of the silicon single crystal structure, j is the current density and .pi.'.sub.63 is the piezo resistance coefficient.
In reality, however, in practical force transducers, stresses other than the stress .sigma..sub.3 which is along the &lt;110&gt; axis are without fail also generated. It thus follows that the actual output .DELTA.V from the device is not completely defined by equation 1 but is instead expressed by equation 2 which contains all six components of stress. ##EQU1##
With this conventional force transducer, the stresses that were generated in addition to .sigma..sub.3, that is, .sigma..sub.1, .sigma..sub.2, .sigma..sub.4, .sigma..sub.5, .sigma..sub.6, were considered to be negligible, and this transducer was not constructed to take into account the detrimental effects of the output .DELTA.V.sub.2 resulting from these stresses. This causes a problem where the negative effects of the output .DELTA.V.sub.2 resulting from stresses other than .sigma..sub.3 are detrimental to the conversion efficiency of the actual device.
The reasons for this are as follows. Table 1 and Table 2 show the value of each piezo resistance coefficient .pi.' for a P-type silicon single crystal structure with a resistance ratio of 7.8 .OMEGA..cm, and an N-type silicon single crystal structure with a resistance ratio of 11.7 .OMEGA..cm respectively. The aforementioned Tables 1 and 2 also show values for the stress created in a conventional type support bed and values for the device output acquired according to equation 1.
TABLE 1 ______________________________________ i 1 2 3 4 5 6 ______________________________________ Piezo resistance .pi.' 61 .pi.' 62 .pi.' 63 .pi.' 64 .pi.' 65 .pi.' 66 coefficient -16.3 -16.3 32.6 0.0 0.0 40.3 .pi.' .sub.6i *1 - - + .largecircle. .largecircle. + Stress .sigma..sub.i - - - + + .about..largecircle. Output .DELTA. V' + + - .largecircle. .largecircle. .largecircle. ______________________________________ .rho. = 7.8 .OMEGA. .multidot. cm *1; .pi.' .sub.6i (.times.10.sup.-12 cm.sup.2 /dyne)
TABLE 2 ______________________________________ i 1 2 3 4 5 6 ______________________________________ Piezo resistance .pi.' 61 .pi.' 62 .pi.' 63 .pi.' 64 .pi.' 65 .pi.' 66 coefficient -17.8 -17.8 35.5 0.0 0.0 -120.1 .pi.' .sub.6i *1 - - + .largecircle. .largecircle. - Stress .sigma..sub.i - - - + + .about..largecircle. Output .DELTA. V' + + - .largecircle. .largecircle. .largecircle. ______________________________________ .rho. = 11.7 .OMEGA. .multidot. cm *1; .pi.' .sub.6i (.times.10.sup.-12 cm.sup.2 /dyne)
As shown in Tables 1 and 2, for the case where the horizontal cross-sectional shape of the support bed is square, for either a P-type or N-type structure the output produced by .sigma..sub.3 is negative. However, on the other hand, the summation of the outputs produced by other stresses (.sigma..sub.1 and .sigma..sub.2 are positive, .sigma..sub.4, .sigma..sub.5 and .sigma..sub.6 are zero), is positive for either a P-type or an N-type structure. This is opposed to the output value produced by .sigma..sub.3, and the overall device output is thus reduced.